What is in dairy processing wastewater sludge (DPS)?

Very little is known about the volumes dairy processing sludge (DPS) produced , the types of DPS and their nutrient and metal profile. Johnstown Castle Environment researchers S.M. Ashekuzzaman & O. Fenton are doing some research

Background to dairy processing sludge (DPS)

As a by-product of the dairy food industry’s wastewater treatment process, significant volumes of “dairy processing wastewater” are applied to agricultural land as an organic fertilizer for crop production. This method of disposal is seen within the dairy industry as only a short to medium term solution for disposal of waste. However, very little is known about the volumes produced, the types of DPS and their nutrient and metal profile.

Volumes and types of DPS

Results from a recent project showed that dairy processing sludge from the dairy food processing industry is a growing bio-based residue in Ireland. There was a 39% increase in volumes observed from 2012 to 2017, totalling up to 126,718 tonnes (wet weight) in 2017.

There are four DPS types:

• Bio-chemically treated activated sludge;
• Lime treated dissolved air flotation processing sludge;
• A combined treatment sludge and
• Anaerobically digested sludge

Profile of DPS types

A two year seasonal sampling regime across the major processing plants in Ireland showed that nutrient contents did not vary seasonally but varied significantly across DPS type and processing plants. This means that you should know what type you are spreading on your land and know what is in it.  

The median N:P:K values (g kg⁻¹ dry weight) were as follows:

• Bio-chemically treated activated sludge (57.2:36.8;7.2);
• Lime treated dissolved air flotation processing sludge (19.5;65.9; 3.9);
• A combined treatment sludge (46; 20; 2.9)
• Anaerobically digested sludge (70.4; 14.6; 6.1)

This means the sludge types are high in nitrogen and phosphorus when compared to cattle slurry and farmyard manure but potassium (K) needs to be balanced in these products.

In addition, sludge samples were found to contain several micronutrients required by plants including copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn). Metal concentrations across the entire sampling archive were significantly lower than the EU soil thresholds.

Future updates will focus on the fertilizer replacement value of DPS.

This work has received funding from Enterprise Ireland and the Dairy Industry Partners in Ireland under Dairy Processing Technology Centre (DPTC) programme. Grant Agreement NumberTC2014 0016.

Check out Twitter @ofenton or @SM_Ashek for more updates